Why are my Mail attachments not going through correctly

All right so this is kind of long. I am currently working on Lion at my work. We send attachments that are saved as JPEGS. We've had trouble before with Windows users not being able to download the files we send, they can see it, but can't download it to their computer. As a way around that, we found that sending the JPEGS without any type in the email would work, picture only. It wouldn't be embedded I think this way and they could download it. I'm not sure if there was an upgrade or what but now when I send it, they can no longer download it and I'm not even sure if they can see it. If I send it to myself or one of my fellow Mac users it comes through as a JPEG icon, not a picture like before, and if I click on it, it says Could not complete your request because an unknown or invalid JPEG marker type is found. I'm not sure what is going on, or what has changed, but it seems like a Mac issue since I can no longer open the files myself. A quick fix would be appreciate as it affects my work. Thanks, and if you need any more info, ask.